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Abstract:

Muscle contraction is brought about by the interaction of the proteins actin and myosin. Xray
diffraction is a useful tool for gaining information about this process. This thesis reports
on the effect of sarcomere length on the X-ray diffraction patterns from active and rigor bony
fish muscle. During contraction, sarcomere I.ength changes are known to slow the
development of tension (Cecchi et al., 1991) and effect X-ray reflection intensities, (Elliott et
al., 1963). Up until now the size of the contractile sarcomere length change in these muscles
was unknown and its effect on the tension and X-ray intensities had been neglected (Harford
and Squire, 1992). Previously, a time lag was observed between the intensity changes of
. the first two major equatorial X-ray reflections (A(10) and A(11Ãƒ?Ã‚Â». This led to a hypothesis
that the low and high force attached crossbridge states are structurally distinct (Harford and
Squire, 1992).
In this project, a sarcomere length measurement and control system was developed. The
contractile performance of the bony fish muscles was also improved. For the first time, the
sarcomere length change during contraction of whole Plaice fin muscle was measured (a
reduction of (3.10Ãƒ?Ã‚Â±0.06)% per sarcomere) and the system could halve this change. This
sarcomere length control was found to significantly increase the rate of tension development.
X-ray data also showed a reduction of over half in the lag between the two intensity changes
(A(10) and A(11Ãƒ?Ã‚Â», providing less clear evidence that the two crossbridge states are
structurally different, a finding closer to that seen in frog muscle (Cecchi et al., 1991).
Using two pre-existing X-ray datasets, the effect of initial sarcomere length on the state
induced in rigor bony fish muscle was also investigated. Evidence from the intensity
distributions on the actin layer-lines and from electron density maps of the muscle crosssection,
(Harford et al., 1994), suggested that in rigor muscles with a longer initial sarcomere
length than the conventional 2.2lJm, a different state was induced in the specimens. This
was characterised by a smaller tropomyosin shift, a different crossbridge labelling pattern
and a different average head shape, possibly closer to that seen in active muscle.